1. Field of the Invention
The invention relates to a method and apparatus for focusing, collimating or expanding a beam of laser light.
2. Description of the Prior Art
It is known that gases transported within a tube can act to diverge, collimate or to focus a beam of light directed along the axis of the tube. It is also known that high intensity light can damage solid lenses. Thus, a lens constructed using a gas with low light absorption is of interest in such circumstances because of its ability to accommodate high light intensities that would otherwise damage solid lenses. Such lenses are an important element in any ground-based or space-based, high-power free electron laser system. However, gas lenses that have been proposed to date tend to introduce aberrations in the optical beam and degrade its optical quality in a way that is difficult to correct, even for low intensity light beams.
For example, a pipe flow gas lens concept has been proposed by Marcuse, et al. of Bell Laboratories (D. Marcuse and S. E. Miller, "Analysis of a Tubular Gas Lens," the Bell System Technical Journal, July 1964, pp. 1758-1782). In this concept, a laser beam is propagated through a cooled rotating pipe which confines a relatively warm flowing gas (FIG. 1a). As shown in FIG. 1b, the radial temperature variation of the gas flowing in the pipe produces a radial refractive index variation which corresponds to a negative optical lens. The use of a warm pipe and a relatively cool gas, as discussed in REF. 1, produces a positive optical lens.
There are several shortcomings associated with this type device. First, the radial variation of the index of refraction is not parabolic in the pipe inlet boundary-layer region. This non parabolic index variation produces aberrations that degrade beam quality. Second, at high optical intensities, these devices are susceptible to distortions caused by heating of the gas and thermal blooming because of the long dwell time of the gas in the laser beam path. The amount of distortion increases as the beam travels along the optic axis. Third, these devices are not scaleable because optical power is reduced in the downstream flow region due to gas-wall temperature equilibration, as indicated in FIG. 1b.
Another device is proposed by McConnel in U.S. Pat. No. 4,402,274. This device is similar to the Marcuse invention in that the laser beam is propagated along the axis of an axisymmetric flow field. However, unlike Marcuse, the radial density gradient is generated by a vortex, rather than by a heating or cooling the tube walls of the containment tube, as is done by Marcuse.
The principal shortcoming of the above aerolens devices which incorporate axisymmetric flow fields is that they do not produce a true parabolic lens profile. Thus, laser beam quality is degraded in passage through these devices.
It is therefor an object of the present invention to generate a gas lens which is characterized by negligible distortion.
It is also an object of the invention to maintain negligible distortion independent of distance along the optic axis.
It is another object of the invention to accommodate high power laser beams with minimum heat absorption and nonlinear optical effects.
It is yet another object of this invention to provide a configuration which is readily scaleable.